My lab pioneered the study of homeostatic plasticity in neocortical neurons and networks. Recently we found that neocortical neurons both in vitro and in vivo have a firing rate set point (FRSP) around which they regulate their average firing. This is accomplished through a set of homeostatic plasticity mechanisms, including synaptic scaling and homeostatic regulation of intrinsic excitability, that act to restore excitability when it is perturbed by changes in sensory drive or synaptic input. We have made significant inroads into understanding the molecular signaling pathways and machinery that accomplishes this homeostatic regulation of firing; in particular, we know that cell-autonomous changes in average neuronal firing that modulate somatic calcium influx lead to changes in signaling through the calcium-dependent kinase CaMKIV, and this in turn produces transcription-dependent homeostatic alterations in the surface expression of ion channels and glutamate receptors. Despite this progress, it remains largely mysterious how neocortical neurons (or any other cell type) build a stable firing-rate set-point out of calcium-dependent signaling pathways. Here we wish to test the central hypotiiesis that FRSP in neocortical neurons is the equilibrium point of opposing calcium-dependent signaling pathways that regulate excitability, and that this basic principle for how to build a stable FRSP wdll generalize to fundamentally different cell types. We have three major aims: i) test the prediction that FRSP can be modulated by altering the strength of specific calcium-dependent signaling pathways, such as CaMKIV; 2) Identify other elements that act in opposition to CaMKIV and together comprise the FRSP, and 3) determine which homeostatic effectors are downstream of CaMKIV to generate homeostatic compensation. This project wdll illuminate the mechanistic underpinnings of a fundamental aspect of neuronal physiology, and through collaborative work the Other Pis on this Program Project.